1. Field of the Invention
The present invention relates to a slip control device of a lock-up clutch.
2. Description of the Related Art
In general, a vehicle provided with a fluid type power transmitting device such as a torque converter and a fluid coupling which transmits output torque of an engine to a transmission is provided with a lock-up clutch for improving fuel efficiency by reducing a fluid loss of torque in the fluid type power transmitting device. The lock-up clutch is arranged in parallel to the fluid type power transmitting device to directly connect the engine to the transmission when fully engaged. However, when the engine is directly connected to the transmission by the lock-up clutch at a low vehicle speed, torque fluctuation of the engine is directly transmitted to the transmission, so that drivability is deteriorated. Therefore, in general, at the low vehicle speed, both improvement in fuel efficiency and inhibition of deterioration in the drivability are satisfied by performing slip control of the lock-up clutch to use the lock-up clutch while allowing the same to slip by a requisite minimum slip amount.
Torque capacity of a fluid type power transmitting device non-linearly changes according to an engine speed, a rotational speed of an output shaft of the fluid type power transmitting device (turbine speed), and a difference in rotational speed between an input shaft and the output shaft of the fluid type power transmitting device (slip speed). Therefore, it is necessary to take a non-linear characteristic of the fluid type power transmitting device into consideration for calculating torque capacity of a lock-up clutch (magnitude of torque transmitted by engagement of lock-up clutch) required for maintaining a slip amount of the lock-up clutch. In such a background, conventionally, the non-linear characteristic of the fluid type power transmitting device is identified by a high-order model and the non-linear characteristic of the fluid type power transmitting device is taken into consideration by using a high-order controller. However, operation to identify the high-order model and operation to design the high-order controller are complicated, so that it takes a lot of time and effort. A coefficient of a high-order filter included in the controller is not directly connected to behavior of an actual machine, so that it is difficult to intuitively comprehend the same, and tuning by using the actual machine is eventually difficult.
Meanwhile, in order to solve such a problem, Japanese Laid-open Patent Publication No. 2005-351329 suggests technology to switch a method of controlling the slip amount of the lock-up clutch for each operating range. Specifically, the technology disclosed in Japanese Laid-open Patent Publication No. 2005-351329 controls the slip amount of the lock-up clutch by open loop control based on a driving state of a vehicle in the operating range in which relationship between a torque capacity coefficient of a torque converter and a speed ratio is non-linear. On the other hand, in the operating range in which the relationship between the torque capacity coefficient of the torque converter and the speed ratio is linear, the technology disclosed in Japanese Laid-open Patent Publication No. 2005-351329 controls the slip amount of the lock-up clutch by feedback control based on a difference between a target slip amount and an actual slip amount. However, comprehensive tuning of the method of controlling for each of a plurality of operating ranges, so that the number of steps is enormous. The comprehensive tuning is not a realistic method when various pieces of operation by a driver, environmental variation, and individual variation are taken into consideration.
There is a need for a slip control device of the lock-up clutch capable of optimally controlling the slip amount of the lock-up clutch over an entire operating range by calculating the torque capacity of the lock-up clutch by a simple configuration without the need of a lot of cost.